st: RE: xtmixed and missing data

I am estimating an xtmixed function while controlling previous year
experience/achievement. How I am using three previous year testing
data to predict the current year's performance. However, I do not want
to exclude individuals in my sample who do not have a score in each of
three years. Currently, when I am estimating the predicted value, it
is dropping all those who do not have a value in each variable.
Is there is a command I could use to allow data to be estimated on the
number of data points an individual has? Overview of my data is as
follows:
Subject # --- Year X Test --- Year X-1 Test ---- Year X-2 Test ---
Year X-3 Test ---- Covariates
Thank you
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